1. Field of the Invention
The present invention relates to a motorcycle and, more particularly, to a swingarm assembly coupled pivotally with a motorcycle frame structure for the support of a motorcycle rear wheel.
2. Description of the Prior Art
In recent years, in an attempt to reduce the weight of the swingarm assembly used to support a motorcycle rear wheel, motorcycles, particularly those for use in motocross, make use of an aluminum alloy as material for the swingarm assembly. It has, however, been found that the sole use of the aluminum alloy as a material for the motorcycle swingarm assembly has its limit to further reduction in weight of the motorcycle swingarm assembly. Accordingly, an attempt has been made to use a motorcycle swingarm assembly of two piece construction including upper and lower swingarm subassemblies such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2003-312577, published Nov. 6, 2003. According to this publication, the lower swingarm subassembly is prepared from a casting of a magnesium alloy whereas the upper swingarm subassembly is prepared from a sheet of an aluminum alloy.
In this known swingarm assembly of the two piece construction discussed above, it is only the lower swingarm subassembly, not the swingarm assembly in its entirety, that is made of the magnesium alloy, which is known to be lighter than the aluminum alloy, and the lower swingarm subassembly is in the form of a cast product having a substantial wall thickness for the sake of physical strength. Accordingly, as compared with the standard motorcycle swingarm assembly made of the aluminum alloy, a substantial reduction in weight of the swingarm assembly would hardly be accomplished. Also, the casting of the magnesium alloy is unable to provide not only a pleasant feeling to texture, but also an aesthetic beauty.